Posts Tagged ‘voltage regulation’

Last time we learned about a new type of transistor called a bipolar transistor and how it controls the flow of electric current traveling from the collector to the emitter within our circuit. We also saw how the bipolar transistor is integrated within a Zener diode voltage regulator circuit to make a new type of circuit called a transistor series voltage regulator.

Now let’s see how this all works by hooking our circuit up to both an unregulated power supply and an external supply circuit as shown in Figure 1.

Figure 1

When voltage VUnregulated is applied to our transistor series voltage regulator circuit by way of an unregulated power supply, electric current flows through RLimiting into the base, B, of the transistor. The transistor senses this current and responds by opening a path for current to flow from its collector, C, to its emitter, E. With this path established, current flows freely from the unregulated power supply, through the transistor’s collector and emitter, on to the output terminal, and finally to the external supply circuit. Total resistance of this circuit is said to be RTotal.

At this point you’re probably wondering why the bipolar transistor base and Zener diode are connected to RLimiting. Next time we’ll conclude our series by seeing how this connection is crucial to the functionality of our transistor series voltage regulator.

We’ve been discussing the Zener diode voltage regulator circuit, its advantages and disadvantages. We learned that the limiting resistor, RLimiting, creates a major disadvantage in the operation of the circuit, effectively acting as a roadblock to restrict current flow. Let’s see how to improve on that.

Figure 1 illustrates a transistor series voltage regulator circuit.

Figure 1

In this circuit the transistor is known as a bipolar transistor. Like the FET we discussed earlier, it has three electrical connections, however on the bipolar transistor the connections are referred to as the collector,base, and emitter. These are labeled C, B, and E in Figure 1.

The bipolar transistor acts as a valve, resting within the main path of current flow. That is, it controls the flow of electric current traveling from the collector to the emitter, as well as the voltage available at the emitter. The transistor is designed so that current flows in one direction only, from collector to emitter. We’ll talk more about that in our next article.

The limiting resistor, RLimiting, is located on a branch of the circuit leading to the Zener diode and the transistor base. Next time we’ll connect an unregulated power supply and external supply circuit to our transistor series voltage regulator. This will enable us to see how placing RLimiting on the branch, rather than along the main current path, results in a major advantage over using the Zener diode voltage regulator alone.

Back when television had barely escaped the confines of black and white transmission there was a men’s clothing store commercial whose slogan still sticks in my mind, “Large and small, we fit them all.” It’s a nice concept, but unfortunately the same doesn’t always apply to electronic power supplies.

Last time we learned that when the electrical resistance changes on an unregulated power supply its output voltage changes proportionately. This makes it unsuitable for powering devices like microprocessor chips, which require an unchanging voltage to operate properly. Now let’s look at another shortcoming of unregulated power supplies, that being how one supply can’t fit both large and small voltage requirements.

Figure 1 shows the components of a simple unregulated power supply.

Figure 1

The diagram illustrates the voltage changes taking place as electric current passes through the supply’s four components, which ultimately results in the conversion of 120 volts alternating current (VAC) into 12 volts direct current (VDC).

First the transformer converts the 120 VAC from the wall outlet to the 12 volts required by most electronic devices. These voltages are shown at Points A and B. The voltage being put out by the transformer results in waves of energy which alternate between a positive maximum value, then to zero, and finally to a maximum negative value.

But we want our power supply to produce 12 VDC. By VDC, I mean voltage that never falls to zero and stays at a positive 12 volts direct current consistently. This is when the diode bridge and capacitor come into play. The diode bridge consists of four electronic components, the diodes, which are connected together to form a bridge and uses semiconductor technology to transform negative voltage from the transformer into positive. The result is a series of 12 volt peaks as shown at Point C.

But we still have the problem of zero voltage gaps between each peak. You see, over time the voltage at Point C of Figure 1 keeps fluctuating between 0 volts and positive 12 volts, and this is not suitable to power most electronics, which require a steady VDC current.

We can get around this problem by feeding voltage from the diode bridge into the capacitor. When we do that, we eliminate the zero voltage gaps between the peaks. This happens when the capacitor charges up with electrical energy as the voltage from the diode bridge nears the top of a peak. Then, as voltage begins its dive back to zero the capacitor discharges its electrical energy to fill in the gaps between peaks. In other words it acts as a kind of reserve battery. The result is the rippled voltage pattern observed at Point D. With the current gaps filled in, the voltage is now a steady VDC.

The output voltage of the unregulated power supply is totally dependant on the design of the transformer, which in this case is designed to convert 120 volts into 12 volts. This limits the power supply’s usefulness because it can only supply one output voltage, that being 12 VDC. This voltage may be insufficient for some electronics, like those often found in microprocessor controlled devices where voltages can range between 1.5 and 24 volts.

Next time we’ll illustrate this limitation by revisiting our microprocessor control circuit example and trying to fit this unregulated power supply into it.

I’m sure you’ve seen the television commercials warning about harmful interactions between prescription medications. By the same token electronic circuitry can also be adversely affected by certain combinations of electrical components, as we’ll discuss in today’s blog.

Last time welooked at a circuit schematic containing an unregulated power supply. This power supply was connected to an external supply circuit containing a number of components such as electric relays, buzzers, and lights. Each of these components has a resistance factor, and combined they have a total resistance of RTotal. We saw that when RTotal increases, the electrical current, I, decreases, and when RTotal decreases, I increases.

In contrast to this increasing/decreasing activity of the total resistance RTotal, the fixed internal resistance of the unregulated power supply, RInternal, doesn’t fluctuate. Let’s explore Ohm’s Law further to see how the static effect of RInternal combines with the changing resistance present in RTotal to adversely affect the unregulated power supply output voltage, VOutput, causing it to fluctuate.

Figure 1

In Figure 1 RTotal and RInternal are operating in series, meaning they are connected together like sausage links. In this configuration their two resistances add together as if they were one larger resistor.

Generally speaking, Ohm’s Law sets out that the current, I, flowing through a resistor in an electrical circuit equals the voltage, V, applied to the resistor divided by the resistance R, or:

I = V ÷ R

In the case of the circuit represented in Figure 1, the resistors RInternal and RTotal are connected in series within the circuit, so their resistances must be added together to arrive at a total power demand. Voltage is applied to these two resistors by the same voltage source, VDC. So, for the circuit as a whole Ohm’s Law would be written as:

I = VDC ÷ (RInternal + RTotal)

But, Ohm’s Law can also be applied to individual parts within the circuit, just as it can be applied to a single kitchen appliance being operated on a circuit shared with other appliances. Let’s see how this applies to our example circuit’s RTotal next week.

Electrical voltage flow and water flow have a lot in common. They’re both affected by fluctuations in supply, fluctuations which can adversely impact both performance and equipment integrity. Take for example a sprinkler that fails to cover a designated section of lawn due to heavy neighborhood demand. Everybody wants to water on the weekend when it’s been 90 degrees all week, and low water pressure is the result. There are times when it’s hard to get a glass of water. By contrast in the winter months, when water demands tend to be lower, water supplies are plentiful. This scenario of varying water pressure is analogous to what sometimes occurs within electric circuits.

In my previous blog article on wall warts, I described the operation of a simple power supply consisting of a transformer, diode bridge, and capacitor. Together, these components converted 120 volts alternating current (VAC) to 12 volts direct current (VDC). The wall wart power supply is fine for many applications, however it is unregulated, meaning if there are any sudden surges in power, such as spikes or dips caused by lightning strikes or other disturbances on the electric utility system, there could be problems.

Take for example a power supply that is used in conjunction with sensitive digital logic chips, like the one used in my x-ray film processor design shown in my last blog article. These chips are designed to run optimally on a constant voltage, like 5 VDC, and when that doesn’t happen input signals can fail to register with the computer program and cause a variety of problems, such as output signals turning on and off at will. In the film processor the drive motor may start at the wrong time or get stuck in an on modality. If power surges are high enough, microprocessor chips can get damaged, compromising the entire working unit.

The output voltage of an unregulated power supply can also vary in response to power demand, just as when sprinklers don’t have sufficient water flow to cover a section of lawn. Demand for power can change within a circuit when electrical components like relays, lights, and buzzers are turned on and off by digital logic chips.

Next time we’ll take a look at a basic concept of electrical engineering known as “Ohm’s Law” and how it governs the variable output voltage response of unregulated power supplies.